1. Field of Invention
The present invention relates to an automatic transmission for vehicles. More particularly, the present invention relates to a planetary gear train of an automatic transmission for vehicles which improves power delivery performance and reduces fuel economy.
2. Description of Related Art
Typically, a planetary gear train is realized by combining a plurality of planetary gear sets, and the planetary gear train including the plurality of planetary gear sets receives torque from a torque converter and changes and transmits the torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed, and therefore a vehicle can have economical fuel mileage and better performance. For that reason, the planetary gear train that is able to realize more shift speeds is under continuous investigation.
Though achieving the same number of speeds, the planetary gear train has a different operating mechanism according to a connection between rotational elements (i.e., sun gear, planet carrier, and ring gear).
In addition, the planetary gear train has different features such a durability, power delivery efficiency, and size depend on the layout thereof. Therefore, designs for a combining structure of a gear train are also under continuous investigation.
In addition, the planetary gear train realizes a plurality of shift-speeds. However, another friction member must be operated after one friction member is released so as to shift to a neighboring shift-speed from a view of shift control.
In addition, a step ratio between the neighboring shift-speeds should be controlled to be suitable according to the planetary gear train.
Currently, four-speed and five-speed automatic transmissions are most often found on the market. However, six-speed automatic transmissions have also been realized for enhancement of performance of power transmission and for enhanced fuel mileage of a vehicle. In addition, eight-speed automatic transmissions and ten-speed automatic transmissions have been developed at a good pace.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.